1. Field of the Invention
The invention relates to a master cylinder primary piston for an automotive vehicle and to vehicle master cylinders equipped with such a piston.
2. Description of the Related Art
Master cylinders known to the state of the art have a primary piston and a secondary piston, generally made of aluminum, both of which are mounted in series along an axial bore in the body of a brake master cylinder, generally made of machined aluminum. Such a master cylinder is described in French patent FR 2,827,244 or U.S. Pat. No. 4,831,916. A push rod is used to actuate the movement of the primary piston. The role of the primary piston is to pressurize a primary pressure chamber and the role of the secondary piston is to pressurize a secondary pressure chamber. Primary and secondary springs tend to push the pistons in the direction opposite this movement, thereby ensuring the increase in pressure.
The master-cylinder bore hole is supplied with brake fluid from two supply access holes that are connected to a brake fluid reservoir. The supply access holes are used to supply the primary and secondary pressure chambers. These holes emerge into annular chambers, annular seals known as cups are provided on either side of the annular chambers.
Brake fluid is supplied to the pressure chambers when the pistons are at rest. The pistons are then in the position shown in FIG. 1. Supply takes place by means of passages provided in the walls of the pistons, which promote communication between the supply access holes and annular chambers and the interior of the primary and secondary pistons, culminating respectively in primary and secondary pressure chambers. Whenever the pistons are axially moved forward, which is the direction of arrow D in FIG. 1, the piston passages cross the seals, thereby isolating the supply chambers and allowing a brake pressure to be established in the primary and secondary pressure chambers.
The master-cylinder assembly is capable of being mounted on a brake-assist servomotor.
Whenever the pistons are moved in the direction of arrow D by the push rod that exerts a selective pressure on the primary piston, cup 4 isolates the primary pressure chamber from the primary access supply hole and cup 6 isolates the secondary pressure chamber from the supply access hole. Whenever the force on the push rod is relaxed, the volume of brake fluid accumulated in the brakes and springs of the master cylinder push the pistons back into rest position. It can happen that, when the push rod is quickly released, the brake fluid contained in the master-cylinder pressure chambers drops below atmospheric pressure due to the action of the primary and secondary springs, which repulse the pistons more rapidly than the flow capacity of the brake fluid in the master cylinder. When the pistons reach rest position, communication between the reservoir at atmospheric pressure and the chambers of the master cylinder is directly established, and there occurs a sudden flow of brake fluid, which generates a noise in the master cylinder known as a “fluid hammer.”
To improve the performance of master cylinders, it is necessary to provide aluminum master-cylinder pistons with specific shapes, which shapes can entail significant additional costs due to the complexity of their manufacture.